Working element with a piston

ABSTRACT

The invention relates to a thermostat assembly of the type having an open ended casing which defines a chamber and has a piston and a deformable bag in the chamber, the piston being sheathed in the bag. The chamber is filled with a thermostatic liquid which exerts pressure to collapse the bag and force the piston out of the chamber when the liquid is heated. The bag and piston have biconvex shaped cross sections instead of circular cross sections as in the prior art.

United States Patent [191 Dyhr-Mikkelsen Sept. 3, 1974 WORKING ELEMENT WITH A PISTON [75] lnventor: Poul Christian Dyhr-Mikkelsen,

Kirke-Horup, Denmark [73] Assignee: Danioss A/S, Nordborg, Sweden [22] Filed: Feb. 2, I973 [2i] Appl. No.: 329,103

[30] Foreign Application Priority Data Mar. 4, 1972 Germany"; 22l055l [52] us. Cl. 60/530, 73/3683 [51] Int. Cl. F03g 7/06 [58] Field of Search 60/23, 530; 73/3683; 62/23 [56] References Cited UNITED STATES PATENTS 2,907,210 10/1959 Riedel 73/3683 3,064,477 ll/l962 Vernet 73/3683 3,149,455 9/1964 Daley et al.... 3,420,105 l/l969 Winter 73/3683 Primary Examiner-Edgar W. Geoghegan Assistant Examiner-H. Burks, Sr.

[ 5 7 1 ABSTRACT The invention relates to a thermostat assembly of the type having an open ended casing which defines a chamber and has a piston and a deformable bag in the chamber, the piston being sheathed in the bag. The chamber is filled with a thermostatic liquid which exerts pressure to collapse the bag and force the piston out of the chamber when the liquid is heated. The bag and piston have biconvex shaped cross sections instead of circular cross sections as in the prior art.

7 Claims, 11 Drawing Figures PAIENTEDSEP3 m4 3,832.850 sum 2 OF 2 WORKING ELEMENT WITH A PISTON The invention concerns a working element with a piston which projects into a chamber, is surrounded by a deformable bag secured by its open end and is displaceable in this bag by the change in volume of a material contained in the chamber.

A working element of this kind is particularly suitable for thermostatic systems, but its use is not limited thereto. It can be employed very generally for converting the change in volume of a material, e.g., of a fluid or of a deformable substance, into a distance of travel. This change in volume may itself be caused by a change in some other physical value, e.g., a change in pressure or temperature. The distance of travel of the piston can likewise be converted into another physical value, e.g., into a pressure and/or a displacement.

It is known practice to use working elements of this kind for refrigerator thermostats; the bag is then made of rubber and acts as a seal, and the material is an expansible substance, e.g., a liquid, which, upon changing volume due to changes in temperature acts on the piston by way of the rubber bag and displaces the piston in the axial direction. The piston of a working element of this kind is of circular cross-section; the bag is likewise of circular cross-section in its at-rest position in which it is expanded by the piston, or which it assumes without the piston and without the application of an external pressure when a resilient material is used for the bag.

When, with this form of piston and bag, a pressure is applied towards the bag by the material surrounding the bag in the chamber, and the piston is displaced from the bag, the walls of the bag finally lie flat against each other. When this happens, the walls of the bag are bent round on to each other through an angle of 180 along two lines located diametrically opposite each other and extending in the axial direction of the bag and piston, so that longitudinal edges are created. Conversely, when the piston executes a stroke in the opposite direction, the bag is bent open through the same angle. This bending is repeated with each stroke of the piston. Consequently the life of the bag is relatively short. A bag is nevertheless used as an additional seal, since the pressure in the chamber is then considerably higher than when there is no bag.

The object of the present invention is to increase the life of this bag in a working element of the initially stated kind.

According to the invention, this object is achieved by making the contour of a cross-section of the bag when in the at-rest position corresponds approximately to the contour of the lens.

This form of bag in the at-rest position, in which it is expanded by the piston or because of the resilience of the material of the bag without external pressure, offers the advantage that the angle at which the intersecting edges disposed in the plane of the cross-section of the bag converge in the at-rest position is less than 180. Since this angle corresponds to the angle of bending of the walls of the bag, the angle of bend is likewise correspondingly smaller. Furthermore, when the bag bends upon opening and closing, the material of its walls is not stressed to the same extent as in the case of a circular cross-section with an angle of bend of 180. The life of a bag having a lens-like cross-sectional contour is accordingly also longer than that of a bag having a circular cross-sectional contour.

Preferably, the contour of a cross-section of the piston likewise corresponds approximately to the contour of a lens.

Normally, the cross-section of the piston is approximately equal to the internal cross-section of the bag when not loaded. The internal cross-sectional area of the bag may however be greater than that of the piston. In both cases the angle of bend of the walls of the bag at points between the longitudinal edges of the bag is likewise smaller as compared with a circular crosssection bag.

lt is well known that a circle has a smaller periphery than any other two-dimensional geometrical figure of equal area. Since the cross-sectional area of the piston is the decisive factor as regards the force whereby the piston is pushed in the axial direction, then in order to obtain the same piston force as is achieved with a piston of circular cross section and the same material pressure in the chamber, the lens-shaped crosssectional area of the piston must be equal to the circular cross-sectional area. Thus, the periphery and, correspondingly, also the surface of the piston of lens-like cross-section are greater than the periphery and the surface of the piston having a circular cross-section of the same size. The greater surface of the piston however leads to a greater frictional force between the piston and the bag for the same pressure of the material in the chamber. The form of the cross-section of the piston and/or of the bag can be suitably selected to keep this increase in frictional force within reasonable limits.

Thus, the intersecting edges in the plane of this crosssection may converge towards the longitudinal edge of the piston or bag at an angle of between approximately 15 and 120. The angle advantageously lies within the range to 90. Preferably it is The median portions of the cross-sectional contour between the above-mentioned longitudinal edges are preferably circular arcs. The portions of the crosssectional contour adjoining these longitudinal edges may be rectilinear.

Favourable conditions of friction are likewise created if the ratio of the length of each portion adjoining a longitudinal edge to the length of the straight line extending this portion and running to the extension of the minor axis of the cross-section lies within the range 0 to 0.6, and is preferably 0.4. The periphery of the piston in the plane of the cross-section may be approximately 5 percent to approximately 50 percent greater than that of a piston of the same but circular crosssectional area.

Advantageously, the periphery ratio is within the range of 1.05 to approximately 1.30 and is preferably 1.10.

In order also to obtain comparable bending conditions at the closed end of a bag in the case of a working element in which the piston tapers longitudinally towards its free end face and the bag tapers towards its closed end, the edge angle at the tip may also be of the same order by magnitude as the above angle and the tip may be flattened.

To avoid an excessively sharp bend at the longitudinal edges of the bag, the longitudinal edges of the piston and of the bag may have a radius of curvature in the cross-sectional plane, the radius of curvature of the piston being greater than or equal to that of the bag. Here, the radius of curvature should have a value of from approximately 0.1 mm to approximately 0.5 mm and preferably approximately 0.3 mm, and a radius of curvature of the bag should lie within the range of approximately 0.05 mm to approximately 0.4 mm, the preferred value being 0.2 mm.

The invention will now be described in greater detail by reference to a known working element and an embodiment in accordance with the invention, illustrated in the annexed drawings, in which:

FIG. 1 a is a longitudinal section through a known working element in the at-rest position,

FIG. 1 b illustrates the same working element as in FIG. 1 a but in the operating position,

FIG. 2 a shows a cross-sectional view on line A A of FIG. 1a,

FIG. 2 b shows a cross-sectional view on line BB of FIG. 1b,

FIG. 3 a illustrates the working element of the invention in a cross-sectional view corresponding to that of FIG. 2 a,

FIG. 3 b illustrates the working element of the invention in a cross-sectional view corresponding to that of FIG. 2 b,

FIGS. 4 and 5 illustrate a cross-sectional form of piston and bag in accordance with the invention,

FIG. 6 illustrates, on a larger scale, a circular portion from FIG. 5,

FIG. 7 a is a side view of a piston in accordance with the invention, and

FIG. 7 b shows a view of the piston turned about its longitudinal axis through 90 from the position seen in FIG. 7 a.

The known working element shown in FIGS. 1 a to 2 b consists of a cylindrical chamber 1, a rubber bag 2, which for the purpose of sealing off the piston 4 is secured by its flange edge to the interior of the chamber wall by means of an annular retainer 3, and a piston 4 of circular cross-section (see FIG. 2 a). The piston 4 is also sealed off at its periphery by a further element 5. By way of a pipe 6, a liquid can be passed into the chamber 1 during operation, or for effecting the initial filling when the thermostatic working element is also to be used as a temperature sensor.

When the liquid in the chamber 1 expands, it displaces the piston 4 from the chamber 1 and the bag 2 as shown in FIG. 1 b, the bag 2 being flat as shown in FIG. 2 b. When the piston 4 executes a stroke in the opposite direction, the bag 2 is expanded again into the form seen in FIG. 2 a in which its cross-section is of circular shape. The angle through which the wall of the bag is bent changes from a 180 (FIG. 2 a) to a, 0 (FIG. 2 b). This large change in angle leads to heavy bending stresses in the wall of the bag in the longitudinal plane, which coincides with the edges formed by bending in the longitudinal direction of the bag. These heavy bending stresses result in reduction of the life of the bag 2. 7

These disadvantages are avoided by the substantially lens-like cross-sectional contours of the bag 7 and piston 8 illustrated in FIGS. 3a and 4 to 7 b. With the lenslike contour, the angle a of the bag 7 and the piston 8 in the at-rest position in which the piston 8 expands the bag 7, is less than 180 so that the wall of the bag only requires to be bent through a smaller angle a. The stressing of the material of the bag is correspondingly reduced especially in the longitudinal plane which coincides with the major axis 9 of the cross-section. How- EVeTI'the stress on the material of the bag is also reduced in the other zones of its cross-section, since here too the angle of bend is smaller than in the case of a circular cross-section.

The piston 8 and the bag 7 are preferably of substantially the same cross-sectional contour and have approximately the same cross-sectional area, so that the bag 7 bears to a large extent on the periphery of the piston as illustrated in FIG. 5.

The portions B of the cross-sectional contour are preferably rectilinear and the portions A circular arcs. Also, the portions A and B merge smoothly with each other. Instead of this, the portions B may be curved inwards or even slightly outwards. Alternatively, the lens contour can be formed by curved portions of a higher order, for example by the portions of parabolic and/or ellipses. The ratio of the length b of a portion B to the length f of the straight line extending the portion B to the extension of the minor axis 10 of the cross-section is preferably 0.40.

The piston 8 preferably has a small radius of curvature r at its longitudinal edge 11 in the cross-sectional plane, this radius being greater than the radius of curvature r of the bag 7 at its inner longitudinal edge 12.

The piston 8 tapers towards its end face and the bag 7 tapers towards its closed end at a corresponding angle a, as illustrated in FIGS. 7 a and 7b.

The particular parameters of the shape of the crosssection of the piston and bag are preferably so selected that no excessively high friction occurs between the piston and the bag.

The following table shows preferred, advantageous and usable parameters.

TABLE of the piston and bag Parameter preferred favourable range usable range .Portion A circular all other curves are resembling an are as stated in of a circle previous column Portion B straight curved inwards or as stated in line slightly outwards previous column Angle a 70 when B depends on B, but is rectilinear may be 15 I20 Periphery 1.10 1.05 1.30 1.05 1.50

ratio Periphery of piston/circular periphery for same crosssectional area TABLE-Continued of parameters of the shape of the cross-section of the piston and bag Parameter preferred favourable range usable range Radius of 0.3 mm 0.2 mm 0.3 mm 0.l mm 0.5 mm

curvature 1'; (piston) Radius of 0.2 mm 0.1 mm 0.2 mm 0.05 mm 0.4 mm

curvature r /r l l l b/f 0.40 0 to 0.60

I claim: wherein the apex angle of one of said cross sections is l. A thermostat assembly comprising a cup shaped casing having an open end and defining a chamber, a deformable bag member disposed in said chamber, said bag member having an open end with a surrounding flange attached in sealing engagement with said open end of said casing, a piston member disposed in and being displaceable in said bag member, said piston member having a free end extending through said open end of said casing, said bag member having a generally elliptically biconvex shaped cross section when in an' unstressed condition, and said casing being charged with a thermostatic liquid.

2. A thermostat assembly according to claim 1 wherein said piston member has a generally elliptically biconvex shaped cross section.

3. A thermostat assembly according to claim 2 between 15 and 120.

4. A thermostat assembly according to claim 2 wherein the apex angle of one of said cross sections is between and 5. A thermostat assembly according to claim 1 wherein one of said biconvex cross sections comprise circular arc portions.

6. A thermostat assembly according to claim 5 wherein said one of said biconvex cross sections includes rectilinear portions.

7. A thermostat assembly according to claim 2 wherein said bag and piston members each tapers in the direction of its free end, said members each having the apex angle of its cross section remain substantially constant over its entire length. 

1. A thermostat assembly comprising a cup shaped casing having an open end and defining a chamber, a deformable bag member disposed in said chamber, said bag member having an open end with a surrounding flange attached in sealing engagement with said open end of said casing, a piston member disposed in and being displaceable in said bag member, said piston member having a free end extending through said open end of said casing, said bag member having a generally elliptically biconvex shaped cross section when in an unstressed condition, and said casing being charged with a thermostatic liquid.
 2. A thermostat assembly according to claim 1 wherein said piston member has a generally elliptically biconvex shaped cross section.
 3. A thermostat assembly according to claim 2 wherein the apex angle of one of said cross sections is between 15* and 120* .
 4. A thermostat assembly according to claim 2 wherein the apex angle of one of said cross sections is between 70* and 90* .
 5. A thermostat assembly according to claim 1 wherein one of said biconvex cross sections comprise circular arc portions.
 6. A thermostat assembly according to claim 5 wherein said one of said biconvex cross sections includes rectilinear portions.
 7. A thermostat assembly according to claim 2 wherein said bag and piston members each tapers in the direction of its free end, said members each having the apex angle of its cross section remain substantially constant over its entire length. 